This invention relates to an apparatus whereby the calorific value or heat of combustion of a test substance may be determined. More specifically, this invention relates to a device having a bomb in which the combustible substance to be tested is ignited. The heat generated by combustion is then determined by comparison to a known standard.
The measurements of heat of combustion or calorific value has for many years been carried out by means of either adiabatic or isoperibol bomb calorimeter systems. In both of these systems, a steel or illium bomb is utilized to containerize the ignition and combustion of a test sample at high pressure. The bomb is immersed in a water filled vessel, having accurate water temperature measuring means therein. The water is circulated within the vessel by means of a stirrer. The vessel and bomb are then placed within a water jacket, an air space forming a gap between the water vessel and the water jacket.
Both of the above prior art systems require elaborate means for controlling the water jacket temperature. With the isoperibol system, it is necessary to maintain the water jacket at a substantially constant temperature and corrections for heat leak must be made. The water jacket in the adiabatic type systems must follow the calorimeter temperature very closely. In these systems, the water jacket must be capable of rapidly adjusting to eliminate temperature differentials between the water jacket and the calorimeter.
In both of these systems the heat of combustion of the tested substance is determined by calculating the change of temperature of the water in the water vessel and multiplying that change by a predetermined calibration constant. The calibration constant is determined by burning a known standard, such as benzoic acid, and observing the temperature change. The ratio of the test substance temperature change to the standard temperature change is then assumed to be directly proportional to the ratio of the heats of combustion.
Because the change in temperature is directly related to the heat capacity of the water in the water vessel and the calorimeter itself, the amount of water in the water vessel must be accurately weighed and measured. Additionally, the accuracy of the heat of combustion, as calculated, could be no greater than the accuracy of the thermometer or temperature measuring device utilized in the determination.
Bomb-type calorimeters are generally utilized to test relatively large samples, measuring energy of the order of 4.times.10.sup.4 Joules. Devices for measuring relatively large energy emissions are termed "macrocalorimeters". An entirely different methodology is implemented in the measurement of extremely small energy emissions, i.e. of the order of 10.sup.-6 to 10.sup.-2 Joules.
The devices employed for measuring these small energy emissions are appropriately termed "microcalorimeters". These devices directly measure the heat emitted from a sample, as opposed to the heat dissipated therefrom to another body. Heat energy is carried away from the test substance by sets of series connected thermocouples, termed thermopiles. The heat thus emitted is dissipated to a substantially infinite heat sink, usually an aluminum block. The thermopiles generate a voltage which is proportional to the temperature gradient across their junctions. Because all or most of the heat is carried through the thermopiles by conduction, the voltage of the thermopiles is directly proportional to the power emitted from the test substance. An integration of this power with respect to time results in the total energy emitted by the test substance.
In a known application of a device of this type, the open-circuit self-discharge heat losses of tiny batteries, as used in the pace-maker industry, may be determined. in these applications, powers as low as 0.1 .mu.W have been measured.
The use of the above-mentioned methodology has heretofore not been expanded beyond the field of microcalorimetry.